The invention relates to an operating method for an exhaust aftertreatment system and an exhaust aftertreatment.
Exhaust aftertreatment systems are used to reduce vehicular emissions. Future legislation for diesel engines of commercial vehicles such as heavy duty trucks, demands to reduce emissions. A particulate filter system can be used to reduce the soot emission of the vehicle. Typically, after a driving time of several days up to several weeks the particle filter is full and has to be regenerated.
An exhaust aftertreatment system is disclosed in EP-A 1-1 882 829 wherein an oxidation catalyst is arrange upstream of a diesel particulate filter. The system further comprises means for detecting the quantity of particulate matter in the filter and a regeneration signal is sent when the amount of particle exceeds a limit. In order to regenerate the filter, relevant engine parameters are controlled such that a desired temperature is reached. Regeneration of the particulate filter can also be manually triggered. To increase the exhaust temperature for regeneration, unburned fuel can be injected into the exhaust gas.
It is desirable to provide a method for operating an exhaust gas aftertreatment system which allows for controlled particle filter regeneration. It is also desirable to provide an improved aftertreatment system.
An operating method of an exhaust gas aftertreatment system is proposed, which system comprises at least a particulate filter for retaining soot from the exhaust gas of an engine and a deNOx catalytic converter for reducing nitrogen oxide in the exhaust gas of the engine. Operation regimes of the particulate filter and of the deNOx catalytic converter are synchronized with respect to each other for performing regeneration of the particulate filter while the catalytic converter provides a nitrogen oxide conversion efficiency above a predetermined limit.
The operation regimes of the particulate filter and of the deNOx catalytic converter can be synchronized so that the particulate filter can be regenerated at favourable operation conditions of the deNOx catalytic converter and the particulate filter. This can be done when the soot level of the particulate filter has reached a predetermined value or at times the deNOx catalytic filter operates in a favourable operation regime. Favourably, the engine can be set to run in a mode with increased output of nitrogen oxide NOx and higher exhaust gas temperatures. Preferably, nitrogen dioxide NO2 instead of oxygen is provided for burning soot in the particulate filter. This will activate a NO2-based regeneration of the particulate filter, wherein NO2 oxidizes the soot accumulated in the particulate filter. After the soot level in the particulate filter is reduced to a desired level, the engine mode can return to normal operation conditions with a normal generation level of NOx and lower exhaust gas temperatures.
The active regeneration of the particulate filter with NO2 can be done when the engine as well as the vehicle is in a running mode. The activation of the NO2-based regeneration of the particulate filter can be triggered even in stand-still conditions while the vehicle is parked. For instance, the driver can activate the regeneration manually by pressing a button or a switch which accordingly sets the engine in a mode with an increased NOx generation and a higher exhaust gas temperature. Preferably, the particulate filter regeneration is done when there is sufficient NO2 in the exhaust gas at a sufficient high temperature.
Advantageously such an operation mode (“heat mode”) of the engine can be enforced by a proper adjustment of exhaust gas recirculation, air mass flow (e.g. by adjusting a turbine geometry of a Variable Geometry Turbine (VGT)), post injection of hydrocarbon into the exhaust gas, intake throttle setting and/or Exhaust Pressure Governor (EPG) device.
Favourably, the heat mode of the engine is controlled in a way to generate exhaust gas temperatures within an upper and a lower limit. Expediently, the upper temperature limit can be set by the deNOx catalyst due to detrimental effects of catalyst aging if the catalyst is exposed to too high temperatures, and the lower temperature limit can be set by the deNOx catalyst due to detrimental effects on emission abatement during particulate filter regeneration.
At the same time, if the operation regime of the deNOx catalytic converter converts the nitrogen oxides NOx with high efficiency, the regeneration of the particulate filter can be emission neutral or at least varied within predetermined limits. Synchronization of the regeneration operation regime of the particulate filter with the operation regime of the deNOx catalytic converter with high NOx conversion, particularly over selective catalytic reduction (SCR), can be triggered by the determination of the NOx conversion over the deNOx catalytic converter. This can be easily done for instance via two NOx sensors, one upstream and one downstream of the deNOx catalytic converter. The NOx sensor upstream of the deNOx catalytic converter can be a virtual sensor, i.e. the “signal” of the virtual is derived from operation parameters of the engine and the components between the engine and the deNOx catalytic converter, or a real sensor. Preferably, the NOx sensor downstream of the deNOx catalytic converter is a real sensor. Expediently, the two NOx sensors can be coupled with a timer and/or a differential pressure sensor determining the soot load of the particulate filter.
Favourably, a risk of a spontaneous soot combustion resulting in very high temperatures and filter damage can be avoided. The particulate filter can be regenerated actively in a controlled manner. Preferably, the filter regeneration is done by oxidizing the soot in the particulate filter with nitrogen dioxide (NO2). Such active soot regeneration can be performed at low temperatures in the range 300° C. to 350° C. In contradistinction to this, an active regeneration with oxygen would require much higher temperatures of above 600° C. which may have a detrimental effect on catalytic converters downstream of the particulate filter.
Advantageously, a higher soot load of the particulate filter can be tolerated as well as longer regeneration intervals without a risk of damaging the particulate filter. Particularly, oxidation of the complete soot load in the particulate filter is not necessary as the particulate filter regeneration can be interrupted when necessary. The particulate filter regeneration can be easily resumed at a later time. In contradistinction to this, regeneration with oxygen instead of NO2 would require oxidizing all soot in the particulate filter generating high exhaust gas temperatures without the possibility to stop regeneration.
The method can be performed at relatively low temperatures. Therefore, components in the exhaust gas aftertreatment system can be prevented from damage caused by high temperatures. Aging of catalytic converters, e.g. a diesel oxidation catalyst, can be reduced. Particularly, aging of components downstream of the particulate filter can be reduced.
Favourably, if an increase of the exhaust gas temperature has to be achieved, hardware for raising temperature in the form of an increased back pressure device, such as a butterfly valve, can minimize or even avoid the necessity of a hydrocarbon injection into the exhaust gas.
According to a favourable method step of an aspect of the invention, the operation regimes can be synchronized with respect to at least one of operation temperature of the particulate filter and/or the deNOx catalytic converter; exhaust gas composition; timing of adjusting at least one of the operation regimes. Favourably, an appropriate temperature range for a proper and efficient operation the deNOx catalytic converter as well as a sufficient amount of NO2 as oxidant for the soot in the particulate filter can be achieved.
According to a favourable method step of an aspect of the invention, dosing of a reductant upstream of the deNOx catalytic converter can be modified such that a nitrogen oxide emission is kept constant or modified within determined boundaries. Advantageously, the regeneration of the particulate filter can be performed virtually neutral with respect to emissions.
According to a favourable method step of an aspect of the invention, an operation temperature of the exhaust gas downstream of the particulate filter and upstream of the deNOx catalytic converter can be limited to an upper temperature tolerable for the deNOx catalytic converter. Aging of the deNOx catalytic converter due to exposure to high exhaust gas temperatures can be reduced.
According to a favourable method step of an aspect of the invention, an operation temperature of the exhaust gas downstream of the particulate filter and upstream of an additional catalytic converter can be limited to an upper temperature tolerable for the additional catalytic converter. Aging of the additional catalytic converter due to exposure to high exhaust gas temperatures can be reduced. Such an additional catalytic converter can be an oxidation catalytic converter for oxidizing residual reductants like hydrocarbons and/or ammonia in the exhaust gas. Such catalytic converter is sometimes also known as clean-up catalytic converter.
According to a favourable method step of an aspect of the invention, the operation regime of the particulate filter and/or the catalytic converter can be compensated for an aging effect of one or more components of the exhaust gas aftertreatment system. Advantageously, the regeneration of the particulate filter and the emissions can be optimized.
According to a favourable method step of an aspect of the invention, the operation regimes of the particulate filter and the deNOx catalytic converter can be synchronized continuously at least during operation of the engine. The particulate filter can be regenerated at times when the deNOx catalytic converter is in a highly efficient mode for conversion of NOx to nitrogen and water. Regeneration can be stopped when the operation conditions of the deNOx catalytic converter starts to be less efficient. Regeneration of the particulate filter can be resumed as soon as the deNOx catalytic converter reaches a high efficient conversion mode again. Averaged over time, the particulate filter can be kept in a state with low soot load.
According to a favourable method step of an aspect of the invention, the operation regimes of the particulate filter and the deNOx catalytic converter can be synchronized on a periodic basis at least during operation of the engine. The regeneration mode of the particulate filter can be performed at well defined times which can be chosen according to desired operation modes or ambient conditions of the vehicle or the like.
According to another aspect of an aspect of the invention an exhaust gas aftertreatment system for performing the inventive method is proposed, wherein a control unit is provided which is adapted to synchronize operation regimes of the particulate filter and of the deNOx catalytic converter with respect to each other for increasing an amount of nitrogen oxide for oxidizing soot in the particulate filter and enhancing the nitrogen oxide conversion in the catalytic converter during a regeneration phase of the particulate filter. Expediently, a controlled regeneration of the particulate filter can be achieved which is virtually neutral for emissions. The particulate filter regeneration can be stopped in a controlled way and resumed if desired. Regeneration can be triggered actively by providing a sufficient amount of nitrogen dioxide in the exhaust gas. The regeneration of the particulate filter can be performed with nitrogen oxide which can oxidize soot at comparatively low temperatures between e.g. 300° C.-350° C. in contradistinction to regeneration with oxygen which is operable at much higher temperatures in the range of 600° C.-650° C. Thus, overheating of the exhaust gas can be avoided.
According to a favourable embodiment of an aspect of the invention, the deNOx catalytic converter can be a selective catalytic reduction catalytic converter. A selective catalytic reduction (SCR) catalytic converter can convert NOx efficiently to nitrogen and water.
According to a favourable embodiment of an aspect of the invention, an oxidation catalyst can be arranged between the particulate filter and the deNOx catalytic converter. The oxidation catalyst can oxidize carbon monoxide and hydrocarbons as well as NO to NO2.
According to a favourable embodiment of an aspect of the invention, the particulate filter can be arranged upstream of the deNOx catalytic converter, thereby protecting the deNOx catalytic converter performance of being influenced negatively by so-called catalyst poisons, which in the case of the SCR-catalyst can be potassium, phosphorus, sulphur, soot, hydrocarbons, etc.
According to a favourable embodiment of an aspect of the invention, the particulate filter can be arranged downstream of the deNOx catalytic converter reducing the thermal mass upstream of the deNOx catalytic converter. This measure allows the deNOx catalytic converter to have a favourable light-off temperature in regard to NOx, conversion in low temperature driving cycles or cold start conditions due.
According to a favourable embodiment of an aspect of the invention, a clean-up catalyst can be arranged downstream of the particulate filter, deNOx catalytic converter and the oxidation catalytic converter. The clean-up catalytic converter can e.g. eliminate a possible amount of ammonia in the exhaust gas which slipped through the deNOx catalytic converter.
The invention can be used, according to aspects thereof, preferably in commercial vehicles such as trucks of the high duty, medium or low duty type.